API Documentation issue in QueryHelpers

The [documentation for `QueryHelpers.AddQueryString`](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.webutilities.queryhelpers.addquerystring?view=aspnetcore-5.0) has wrong signatures.

Specifically,
* [`AddQueryString(String, IDictionary)`](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.webutilities.queryhelpers.addquerystring?view=aspnetcore-5.0#Microsoft_AspNetCore_WebUtilities_QueryHelpers_AddQueryString_System_String_System_Collections_Generic_IDictionary_System_String_System_String__) should be `AddQueryString(String, IDictionary)` (with nullable dictionary values)
* [`AddQueryString(String, IEnumerable>)`](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.webutilities.queryhelpers.addquerystring?view=aspnetcore-5.0#Microsoft_AspNetCore_WebUtilities_QueryHelpers_AddQueryString_System_String_System_Collections_Generic_IEnumerable_System_Collections_Generic_KeyValuePair_System_String_System_String___) should be `AddQueryString(String, IEnumerable>)` (with nullable values in the KVP)

It seems these types changed to be nullable with ASP.NET Core 5.0, and I'm referring to the 5.0 documentation.